The present invention relates to a hydraulic control system for a continuously variable belt-drive automatic transmission for a motor vehicle having a torque converter provided with a lockup clutch, and more particularly to a system for controlling the lockup clutch and line pressure in a hydraulic circuit for the transmission.
In a continuously variable transmission provided with a torque converter, torque transmitted to the transmission is multiplied by the torque converter. On the other hand, when a lockup clutch provided in the torque converter is engaged, the engine torque is directly transmitted without multiplying. In order to prevent slipping of the belt on the pulleys, the line pressure which is controlled in dependency on the transmitted torque is regulated either for the lockup state or for the torque converter operating state.
Japanese Patent Laid Open 57-161360 (U.S. Pat. No. 4,478,105) discloses a system in which pressure of oil from an oil pump is adjusted by a line pressure control valve for controlling drive and driven pulleys of the transmission. The line pressure control valve produces a control pressure for controlling a torque converter, a lockup clutch, a lockup control valve, and a clutch for a selector device. As the pressure in a drive pulley cylinder increases after the starting of the changing of the transmission ratio, a release side oil chamber of the torque converter is drained to engage the lockup clutch. During the release of the lockup clutch, the pressure applied to the release side oil chamber is applied to the line pressure control valve to increase the line pressure.
However, in the control system, since the lockup clutch is controlled by the pressure applied to the drive pulley, the torque converter can be locked only after the start of the changing of the transmission ratio. Accordingly, when the rotational speed of the drive pulley is decreased, the lockup clutch is disengaged although the lockup of the torque converter may be preferable for the driving condition. Accordingly, the lockup range decreases. Additionally, the oil in the release side chamber is applied to the line pressure control valve in order to control the line pressure. However, the oil in the release side flows constantly, so that the pressure thereof is subject to vary. Thus, the line pressure cannot be increased with accuracy.